Such support jacks are often mounted on semitrailers or trailers in general and they support them especially in a noncoupled condition. The support jacks are made from rectangular tubes, which are inserted one into the other and can move relatively to each other, such as is known for example from EP 1 104 369 B1.
Furthermore, support jacks are used for silo vehicles, in which a silo container with vehicle stationary is set up from a lying to a nearly upright position for emptying. In this upright position, the vehicle is quite susceptible to tipping over, so that it is necessary to prop up the rear of the vehicle, preferably with support jacks arranged on either side. During the extending of the support jacks, the operator must make sure that the ground is as level as possible, with sufficient carrying capacity, and may have to provide underpinning on the ground in the region of the support jack so that the vehicle is stable and cannot tip over.
At times it may even happen that the operator by accident extends only one of the two support jacks.
The operator can scarcely determine whether the support jacks find a sufficient abutment on the ground beneath them. In an extreme situation, the support jack will sink under the load of the vehicle into the ground. Then the vehicle is no longer sufficiently supported and it may tip over.